1. Field of the Invention
The present invention relates to a gas sensor for measuring the concentration of a gas to be detected, which is contained in a gas under measurement, such as exhaust gas discharged from an internal combustion engine or the like.
2. Description of the Related Art
Conventionally, an oxygen sensor having a tubular element as described in Japanese Patent Application Laid-open Nos. 9-184822 and 11-295263 has been known and used as a gas sensor for measuring the concentration of a gas to be detected which is contained in a gas under measurement. Further, an oxygen sensor described in German Utility Model Publication No. 29803046 is provided with a protection cap for transportation. When the oxygen sensor is transported, the protection cap is attached to the oxygen sensor.
3. Problems Solved by the Invention
In the above-described conventional gas sensor, an element 4 for detecting a gas is inserted into and accommodated within a metallic shell 3; and in order to protect the element 4, a protector 2 is provided at the axial tip end of the gas sensor so as to cover the element 4. When the gas sensor is transported, a protection cap 1 made of resin is provided around the protector 2 to thereby protect the element 4. The protection cap 1 prevents splashing of water on the element 4 and prevents the element 4 from coming in contact with outside air during transportation and causing deterioration in properties. An annular projection 101 is provided on the inner surface of the protection cap 1 in order to prevent, to the extent possible, contact between outside air and the element 4. Further, in order to prevent contact between outside air and the element 4 via communication holes 201, which are provided to allow a detection gas to contact the element 4 during use of the gas sensor, the space around the communication holes 201 is hermetically sealed.
However, since a seizure-prevention agent 303 (NEVER SEEZE: Product of BOSTIC, etc.) is applied to a male-screw portion of the gas sensor in order to prevent seizure of the male-screw portion, which would otherwise occur upon attachment to an exhaust pipe, when the protection cap 1 is attached to the gas sensor, a solvent contained in the seizure-prevention agent 303 evaporates and fills the interior of the protection cap 1, thereby adversary affecting the characteristics of the element. This is because even when the solvent fills the interior of the protection cap 1, the solvent is not replaced with outside air due to presence of the protection cap 1.
Further, since an airtight seal must be provided between the protection cap 1 and the protector 2, the inner diameter of the projection must be changed in accordance with the size of the protector 2. Therefore, various types of protection caps 1 must be prepared and used selectively in accordance with the type of a sensor to be protected.
The present invention has been accomplished in order to solve the above-described problems. It is therefore an object of the present invention to prevent the element characteristics of a gas sensor from being deteriorated by a solvent evaporated from the seizure-prevention agent 303, which deterioration would otherwise occur when the protection cap 1 is attached to the gas sensor before transportation thereof, thus causing the interior of the protection cap 1 to be filled with the evaporated solvent.
In order to achieve the above-described object, the present inventor employed the following configuration. Each of support portions 102xe2x80x94which are adapted to come into elastic contact with a protector side surface 202 to thereby support a protection cap 1 relative to the protector 2xe2x80x94projects from a tubular-portion inner surface 161 of the protection cap 1 in such a manner that the space between the protector 2 and the tubular portion 106 of the protection cap 1 attached around the protector 2 is not partitioned axially; and an air-outlet hole 103 is formed in the protection cap 1 in order to release a solvent evaporated from a seizure-prevention agent 303. This structure prevents the solvent evaporated from the seizure-prevention agent 303 from filling the interior of the protection cap 1, to thereby prevent the solvent from adversely affecting the element characteristics.
Since the protection cap 1 of the present invention is designed such that gas in the vicinity of the element is replaced with outside air, the element is affected slightly by the outside air. However, since the tubular portion 106 of the protection cap 1 is formed to cover the protector 2, direct contact with water can be prevented to some degree.
Each of the support portions 102 may assume a shape of a plate that extends radially from the tubular-portion inner surface 161 toward the center axis of the sensor and reaches the protector side surface 202. This plate-shaped configuration enables a greater portion of the area between the protection cap 1 and the protector 2 to be maintained unclosed. Thus, the ventilation efficiency is improved further. Further, when the protection cap 1 is formed of a soft resin, the protection cap 1 can be used for a different gas sensor whose protector 2 has a larger diameter, because the plate-shaped support portions 102 projecting from the tubular-portion inner surface 161 easily deform, as shown in FIG. 3. The material of the protection cap is not limited to resin, and may be composed, for example, of paper. Further, a resin having elasticity such as rubber is preferred because use of such an elastic resin facilitates the work of attaching the protection cap to the gas sensor and prevents the protection cap from accidentally coming off the gas sensor.
The air-outlet hole 103 is preferably provided at the tip end of the protection cap 1. In this case, water does not splash onto the element even when communication holes 201 are formed on the protector side surface 202. When a communication hole 201 is formed at the tip end surface of the protector 2, the air-outlet hole 103 is preferably formed at the tubular portion 106 of the protection cap 1, as shown in FIG. 6. However, when the communication hole 201 is small, the air-outlet hole 103 may be provided on the tip end surface of the protection cap 1. At first glance, provision of the communication hole 201 in the vicinity of a region at which the seizure-prevention agent 303 is applied seems to provide good air ventilation. However, in this case, air in the vicinity of the element may not be replaced with outside air. In order to solve this problem, the air-outlet hole 103 of the protection cap 1 is provided in the vicinity of the communication hole 201 of the protector 2 in order to promote replacement of air in the vicinity of the element with outside air. The communication hole 201 of the protector 2 and the air-outlet hole 103 of the protection cap 1 are preferably arranged such that the element 4 is not present on a straight line passing through both holes, because this arrangement prevents splashing of water onto the element 4.